Manufacture of plastic containers

Abstract

The present invention relates to an improvement in a method for thermoforming of containers from thermoplastic materials in which method the rim of the container is formed as an integral part of the pre-form from which the container is thermoformed. The pre-form is obtained by introducing thermoplastic material into a closed space and then mechanically compressing the material. The improvement of the present invention relates to improved uniformity of flow of the thermoplastic material into the closed space, the use of a closed space defined by surfaces having a surface roughness in the range of 50-125 μm rms and to a modified sequence of steps for forming the pre-form in which the compression of the thermoplastic material is commenced prior to the completion of the introduction of the material into the closed space. The process may be used in the manufacture of containers, e.g., for the packaging of margarine, from polyethylene or polypropylene.

We claim: 
     
       1. Improvement in the process for manufacture of plastic containers comprising A. forming a pre-form of thermoplastic material in a transfer means,   B. applying the transfer means carrying the pre-form to a mould having a cavity corresponding to the shape of the container,   C. forming the pre-form into a container in the mould cavity, and   D. separating the container from the cavity, the improvement comprising (a) forming the pre-form by introducing into the closed space between opposite sides of the transfer means a predetermined amount of a thermoplastic material selected from the group consisting of polyethylene and polypropylene, the material being introduced through a passage of at least 0.50 cm diameter in one side of the transfer means, and the closed space being defined by surfaces having a surface roughness in the range of 50-125 μm rms, and (b) mechanically compressing the thermoplastic material within the closed space by applying pressure from the other side of the transfer means prior to completion of the introduction of the material into the closed space, and continuing the mechanical compression after the passage is closed until the closed space is completely filled, the thermoplastic material being maintained in a state of flow in the closed space at all times between the commencement of introduction of the material into the closed space and the completion of the mechanical compression of the material.   
     
     
       2. The method of claim 1 in which the thermoplastic material is polypropylene. 
     
     
       3. The method of claim 1 in which the thermoplastic material is polyethylene. 
     
     
       4. The method of claim 3 in which the thermoplastic material is polyethylene of a density in the range 0.940-0.970 g/cm 3  and a melt index in the range 0.4-1.0. 
     
     
       5. The method of claim 4 in which the polyethylene is a homopolymer of ethylene. 
     
     
       6. The method of claim 4 in which the polyethylene is a copolymer of ethylene and a higher α-olefin having 4-8 carbon atoms. 
     
     
       7. The method of claim 1, claim 2 or claim 4 in which the surface roughness is 85-95 μm rms.